I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for measuring timing of signals received from multiple base stations in a CDMA communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM.xe2x80x9d A specific CDMA system is disclosed in U.S. Patent No. 6,574,211, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,xe2x80x9d issued Jun. 3, 2003. These patents are assigned to the assignee of the present invention and incorporated herein by reference.
A CDMA system is typically designed to conform to one or more standards. Such standards include the xe2x80x9cTIA/EIA/IS-95 Remote terminal-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 25.212, 25.213, 25.214, 25.133, 25.305, 25.331 and 3G TR 25.926 (the W-CDMA standard), and the xe2x80x9cTR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systemsxe2x80x9d (the cdma2000 standard). New CDMA standards are continually proposed and adopted for use. These CDMA standards are incorporated herein by reference.
A CDMA system can be operated to support voice and data communication. During a particular communication (e.g., a voice call), a remote terminal may be in active communication with one or more base stations, which are typically placed in an xe2x80x9cactive setxe2x80x9d of the remote terminal. The remote terminal may also receive signals from one or more other base stations for other types of transmission such as, for example, pilot, paging, broadcast, and so on.
The CDMA system can be designed with the capability to determine the position of a remote terminal. In fact, the Federal Communications Commission (FCC) has mandated support for an enhanced emergency 911 (E-911) service whereby the location of a remote terminal in a 911 call is required to be sent to a Public Safety Answering Point (PSAP). For position determination, the arrival times of the transmissions from a number of base stations are measured at the remote terminal. The differences between the signal arrival times can then be computed and translated into pseudo-ranges, which are then used to determine the position of the remote terminal.
Various challenges are encountered in measuring the signal arrival times. For example, in a wireless communication system, the mobility of the remote terminal may affect the accuracy of the arrival time measurements, if these measurements are not made close in time. Also, the arrival times are typically measured based on the internal timing of the remote terminal, which may be continually adjusted to track the timing of one of the base stations with which the remote terminal is in communication. The remote terminal""s movement and the variation (and uncertainty) in its time reference can impact the accuracy of the arrival time measurements, which may in turn translate to a less accurate estimate of the position of the remote terminal.
Accordingly, techniques that can be used to improve the accuracy of the arrival time measurements, which may lead to improved accuracy in the estimated position of the remote terminal, are highly desirable.
The invention provides various techniques to more accurately measure the arrival times of transmissions received at a remote terminal from a number of base stations. In accordance with one aspect of the invention, unassigned finger processors are used to process and measure the arrival times of transmissions from base stations not in the remote terminal""s active set. In accordance with another aspect of the invention, if no finger processors are available for assignment to base stations not in the active set, the arrival times can be measured in the time period between updates of a reference clock used for the measurements. This can reduce the adverse effect due to slewing of the reference clock as it is adjusted to track the timing of one of the base stations. To reduce the adverse effect due to movement of a mobile remote terminal, the arrival times can be measured within as short a time window as possible.
An aspect of the invention provides a method for determining a position of a remote terminal in a communication system. In accordance with the method, a first set of one or more base stations in active communication with the remote terminal is identified and each base station in the first set is assigned at least one finger processor of a rake receiver. A second set of one or more base stations not in active communication with the remote terminal is also identified and an available finger processor is assigned to each of at least one base station in the second set. A (signal arrival) time measurement (e.g., an SFN-SFN measurement, as defined by W-CDMA standard, or a Pilot Phase measurement, as defined in IS-801) is then performed for each base station assigned with at least one finger processor. Outputs indicative of the time measurements obtained for the assigned base stations are then provided (e.g., to the system) for further processing. Since the finger processors operate in parallel, the measurement can be performed at approximately the same instance in time. This greatly improves the usability of the measurements for position location techniques.
To determine the position of the remote terminal, the arrival times for the earliest arriving multipaths for three or four base stations and/or satellites can be measured.
Various aspects, embodiments, and features of the invention are described in further detail below.